1. Field of the Invention
The present invention relates to a magnetoresistive element and a magnetic memory.
2. Related Art
In recent years, many solid-state memories for recording information based on novel principles have been suggested. Among those solid-state memories, attention has been drawn to magnetoresistive random access memories (MRAM) as solid-state magnetic memories that utilize the tunneling magnetoresistive effect. The MRAMs are characterized by storing data according to the magnetization state of each MTJ (Magnetic Tunnel Junction) element.
In a MRAM that performs writing by inverting the magnetization direction of the storage layer of each MTJ element in the magnetic field caused by a current flowing through a wiring line provided in the vicinity of each MTJ element, the coercive force Hc of each MTJ element in principle becomes larger as the MTJ element is made smaller in size. Therefore, as the size of each MTJ element is reduced, it is necessary to strengthen the magnetic field induced by the current flowing through the wiring line. As a result, it is difficult to produce high-density memories.
To overcome this problem, a MRAM that utilizes a method for writing by inverting the magnetization direction of the storage layer of each MTJ element through a spin momentum transfer (SMT) has been suggested (see U.S. Pat. No. 6,256,223, C. Slonczewski, “Current-driven excitation of magnetic multilayers”, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, VOLUME 159, 1996, pp. L1-L7, and L. Berger, “Emission of spin waves by a magnetic multilayer traversed by a current”, PHYSICAL REVIEW B, VOLUME 54, NUMBER 13, 1996, pp. 9353-9358).
In the magnetization reversal through a spin momentum transfer (hereinafter referred to as the spin injection), the current Ic required for the magnetization reversal is determined by the magnetic anisotropic energy (KuV) of the MTJ element. Accordingly, if the area of the MTJ element is reduced, the injection current Ic required for the magnetization reversal through the spin injection can be reduced. Compared with the above mentioned method for writing by utilizing the magnetic field induced by a current, the write current becomes smaller as the size of the MTJ element becomes smaller. Accordingly, excellent scalability can be expected.
However, the spin injection efficiency of MTJ elements today is not particularly high, and the current IC required for a magnetization reversal is expected to become lower.
When the MTJ elements are used in a high-density memory, the memory structure is strongly expected to have a simple structure and require a simple manufacturing process, and the current is expected to become lower.